The present invention relates to an insulation system for use in windings of a dynamoelectric machine. In particular it relates to an insulation comprising inner and outer layers having differing permittivities to create a more advantageous stress distribution within the dielectric.
Insulation systems for large AC dynamoelectric machines are under constant development to increase the voltages at which these machines operate while at the same time minimizing the thickness of the insulating material.
In such insulation systems it is common to utilize mica in a variety of forms from large flake dispersed on a backing material, to the product known as mica paper. While the low tensile strength of mica paper does not lend itself to use in such insulation systems, mica paper has superior corona breakdown resistance countering the coronal discharge occurring in high voltage windings that tends to shorten the life of the insulation. To compensate for the low tensile strength of the mica paper, the mica paper is bonded to glass fibers which also tends to prevent the shedding of mica flakes from the mica tape during a taping operation.
More recently a corona resistant polyimide and composite insulation tape has been employed in the insulation systems. This tape has exceptional insulation qualities and good corona discharge resistance. This film may be used independently or as a backing on a mica paper, glass fiber composite tape. The addition of enhanced corona resistant tape insulation provides an insulation system which is electrically more enhanced than standard systems.
However, the magnitude and profile of the local electric field within the groundwall insulation has not been considered to date in the development of insulation systems and tapes for the groundwall. This electric field generated in the groundwall insulation as a result of the high voltage applied to the conductor has a direct effect on the insulation life. As various initiatives are in place to reduce the groundwall insulation thickness it should be understood that the effect of the electric field as it is distributed across the groundwall also has an effect on the performance of the insulation system and the life of the groundwall insulation system. Accordingly, there is a need to develop a groundwall insulation system for use in windings for dynamoelectric machines that takes into consideration the effects of the localized electric field generated in the groundwall insulation as a result of the voltage difference across the insulation.
In accordance with the present invention there is provided an insulation system for use in windings of dynamoelectric machines that results in a graded or sharp increase in the electric field as distributed through the insulation from the interior of the insulation adjacent the conductor bar or conducting elements to the outer armor or grounded armor of the insulation.
It should be understood that the term xe2x80x9cgraded increasexe2x80x9d in the electric field relates a significant change in the electric field profile across a cross section of the groundwall insulation. In accordance with the present invention, the electric field profile across a xe2x80x9cflatxe2x80x9d cross section exhibits a sharp step increase part way across the cross section compared with a flat electrical field profile in the past. With respect to a corner section of the insulation, the electric field profile gradually decreases away from the conductors and again exhibits a sharp step increase part way across the corner cross section.
To accomplish the forgoing aspect of the graded change in the electric field profile of the insulation of the present invention, there is provided an insulation system that has a conductor of a dynamoelectric machine which is insulated with layers of insulation. The insulation has a first inner layer of insulation and a second layer of insulation outer relative to the first inner layer. Both the first and second layers of insulation have predetermined thickness so as to provide for the proper insulation characteristic needed for the insulation itself. However, the permittivity of the first inner insulation layer is chosen to be greater than that of the second insulation layer such that the electric field in the second insulation layer increases sharply at the juncture between the first inner and second layers of insulation.
It has been determined that by providing for a relatively higher permittivity on the inner layer, the electric field adjacent the conductor has a reduced magnitude. While the overall electric field distributed across the insulation may not be less, it should be understood that the magnitude of any sharp occurrences of the electric field in the insulating layer adjacent the conductor are reduced. This is a considerable undertaking because the insulation is designed and developed for its weakest areas in the insulation which occur at the corners of the insulation adjacent the conductors where the highest magnitudes of electric field have been experienced in the past. Thus by reducing this magnitude in electric field, the requirements for the thickness of the insulation is reduced thereby minimizing the thickness of the insulation while not adversely effecting the voltages carried by such conductors or the insulation life. It should be understood that in accordance with the present invention it is envisaged that these conductors carry voltages in the order of 4 kV and greater.
It is also envisaged that in alternative embodiments of the present invention the insulation may comprise more than two layers of insulation applied over each other in succeeding layers where each succeeding layer has a permittivity less than the preceding layer of insulation.
A preferred application of the insulation system of the present invention is as a groundwall insulation for conductors in the winding of a dynamoelectric machine carrying voltages of 4 kV and greater. In applications where the voltage is in the order of 13.8 kV, the thickness of the groundwall insulation is in the order of 3.2 mm.
In accordance with a preferred aspect of the present invention there is provided a groundwall insulation for use on a conductor of a dynamoelectric machine that has a graded electric field profile across the groundwall insulation. The groundwall insulation comprises a first inner insulation layer and a second outer insulation layer. The first inner insulation layer is applied over the conductor and has a first predetermined thickness and first predetermined permittivity. The second outer insulation layer is applied over the first inner insulation layer and forms a juncture therewith. The second outer insulation layer has a second predetermined thickness and second predetermined permittivity wherein the second predetermined permittivity is less than the first predetermined permittivity of the first inner insulation layer creating the graded increase in the electric field in the groundwall insulation at the juncture of the first inner and second outer insulation layers.